There is currently a great deal of concern about the health effects of nano-particles and micro-particles emitted unintentionally into the air. For example, a considerable increase in respiratory illness and allergies in the UK in recent years has been associated in part with particles emitted by diesel engines and other combustion processes. Whilst the main focus has been on diesel emissions, attention is turning to other potential sources such as power generation using fossil fuels, incineration, nuclear power generation and aircraft emissions. All heavy industries involving processes emitting fumes have potential problems with the emission of aerosol particles. Such processes include smelting, firing, glass manufacture, welding, soldering, nuclear power generation and incineration. There is also concern amongst consumer companies that use of enzymes in washing powders, powder coatings and fibres used in disposable nappies and other products could cause problems. In addition, the US EPA is becoming increasingly concerned about gasoline engine emissions.
Nano-particles and nano-objects are known to produce toxic effects. For example, they can cross the blood-brain barrier in humans and gold nano-particles can move across the placenta from mother to fetus. Early studies with PTFE (polytetrafluoroethylene) particles around 20 nm in diameter showed that airborne concentrations of a supposedly inert insoluble material lower than 50 μg/m3 could be fatal to rats.
In addition to concerns from a health perspective, the elimination or control of airborne particles is important in maintaining standards in the many thousands of clean rooms in the micro-electronics, pharmaceutical, medical, laser, and fibre optics industries.
Small particles can be classified as shown in Table 1 below.
TABLE 1Aerodynamic Equivalent ParticleTermSize Range (dp = particle size)Dustdp > 10 μmCoarse particles2.5 μm < dp < 10 μmFine particles100 nm < dp < 2.5 μmNano-particles or1 nm < dp < 100 nmultrafine particles
The term “nano-particles” is used to refer to particles having an aerodynamic particle size in the range from 1 nm to 0.1 μm.
For spherical particles, the aerodynamic particle size is the geometric diameter of the particle. Real particles in the air often have complicated shapes. For non-spherical particles, the term “diameter” is not strictly applicable. For example, a flake or a fibre has different dimensions in different directions. Particles of identical shape can be composed of different chemical substances and have different densities. The differences in shape and density cause considerable confusion in defining particle size.
The terms “aerodynamic particle size” or “aerodynamic diameter” are therefore used in order to provide a single parameter for describing real non-spherical particles having arbitrary shapes and densities. As used herein, the term “aerodynamic diameter” is the diameter of a spherical particle having a density of 1 g/cm3 that has the same inertial property (terminal settling velocity) in the air (at standard temperature and pressure) as the particle of interest. Inertial sampling instruments such as cascade impactors enable the aerodynamic diameter to be determined. The term “aerodynamic diameter” is convenient for all particles including clusters and aggregates of any forms and density. However, it is not a true geometric size because non-spherical particles usually have a lower terminal settling velocity than spherical particles. Another convenient equivalent diameter is the diffusion diameter or thermodynamic diameter which is defined as a sphere of 1 g/cm3 density that has the same diffusivity in air as a particle of interest.
The investigation and monitoring of aerosol particles in the atmosphere has been hampered by a shortage of instruments which can measure in the wide particle size range but which are sufficiently inexpensive, robust and convenient to be used on a widespread basis.
Instruments for measuring and selecting aerosol particles can be based upon the electrical mobility of the particles; see for example: Flagan, R. C. (1998): History of electrical aerosol measurements, Aerosol Sci. Technol., 28(4), pp. 301-380. One such instrument is a Differential Mobility Particle Sizer (DMPS) which can be used to determine the size distribution of particles in an aerosol. A DMPS consists of a Differential Mobility Analyzer (DMA), which transmits only particles with a certain size, and a Condensation Particle Counter (CPC), which counts the particles.
A Differential Mobility Analyzer typically comprises a chamber having an inlet and an outlet for an aerosol gas sample and an inlet and an outlet for clean carrier gas (“sheath air” or “sheath gas”) which carries the aerosol gas sample along the chamber. The chamber contains a pair of electrodes of opposing polarities towards which charged particles in the aerosol sample are attracted. In some DMA devices, a concentric electrode arrangement is used in which a rod-like electrode is arranged along the centre of the chamber and a cylindrical outer wall of the chamber functions as the other electrode. In other DMA devices, the electrodes can be a pair of opposed electrode plates.
In use, the aerosol sample is introduced into the chamber through the inlet and is carried along the chamber by the sheath air towards the aerosol gas outlet. As the aerosol passes along the chamber, charged particles in the aerosol are attracted towards one or other of the electrodes. The extent to which the path of the charged particles deviates from a line between the aerosol inlet and aerosol outlet will depend on the electrical mobility of the particles, the potential applied to the electrodes and the flow rate of the sheath air. The electrical mobility of the particles is a function of the size of the particle and its charge. Thus, at a given electrode potential, particle charge and sheath air flow rate, smaller particles will be attracted more readily to an electrode than larger particles. Therefore, by varying the potentials of the electrodes, it is possible to select which size fraction of the aerosol sample is allowed to reach the aerosol outlet. By varying the potentials at the electrodes and repeating measurements at each potential, it is possible to obtain a particle size distribution for the aerosol.
The particles present in aerosol samples taken from the atmosphere or elsewhere will typically contain both charged and uncharged particles in random and unknown proportions. Therefore, prior to entering the chamber, the aerosol gas sample is generally passed through a charging device which imparts a charge to the neutral particles in the aerosol and re-charges or adjusts the charge of particles in the aerosol that are already charged. The charging device is set up to apply a predetermined and consistent charge to the particles.
Normally, a DMA can be used to measure particle sizes of up to about 500 nm or sometimes, by using a longer DMA column, particles having a size up to about 1,000 nm. However, in many practical situations, it is necessary or desirable to measure particles whose sizes may range from a few nanometers up to ten micrometers (defined as PM10). In this wide range, the mobility of the particles varies over several orders of magnitude and, in practice, it is extremely difficult to measure such wide ranges of particle sizes using a single DMA device.
It would undoubtedly be advantageous to be able to measure particle size distributions over a wide range of particle sizes and attempts have been made to solve this problem by carrying out the measurement using two devices, namely an electric mobility analyzer and an impactor. The electric mobility analyzer is first used to measure the size distribution of small particles and then the aerosol is directed to an impactor to determine the size distribution of larger particles. An example of this approach is disclosed in U.S. Pat. No. 7,140,266. U.S. Pat. No. 7,140,266 describes a device in which an electric mobility analyzer and an impactor are connected to each other in such a way that the bottom plate of the mobility analyzer is simultaneously used as the inlet part of the impactor. However, the device of U.S. Pat. No. 7,140,266 suffers from several potential drawbacks. Firstly, the device requires a large pump for the impactor which means that the device would be difficult to build as a portable instrument. Secondly, a cascade impactor classifies particles according to their aerodynamic diameters which may differ from their diffusion diameters, and consequently, the particle size distributions obtained by combining the results obtained from the impactor and the DMA may be rather difficult to interpret.
US 20060266132 discloses a multi-stage differential mobility analyzer for aerosol measurements which includes a first electrode or grid including at least one inlet or injection slit for receiving an aerosol including charged particles for analysis, and a second electrode or grid spaced apart from the first electrode. The second electrode has at least one sampling outlet disposed at a plurality of different distances along its length. A volume between the first and the second electrode or grid between the inlet or injection slit and a distal one of the plurality of sampling outlets forms a classifying region. At least one inlet or injection slit in the second electrode receives a sheath gas flow into an upstream end of the classifying region. Each sampling outlet functions as an independent DMA stage and classifies different size ranges of charged particles based on electric mobility simultaneously. The analyzer disclosed in US 20060266132 enables the measurable particle size range to be extended but a disadvantage is that the dimensions of the apparatus are necessarily increased, thereby mitigating against miniaturization and the construction of portable versions of the instrument.
Some of the problems involved in the measurement of larger particle sizes using differential mobility analyzers can be illustrated by reference to the schematic representation of a known type of DMA shown in FIG. 1.
FIG. 1 is a schematic side sectional view of a planar DMA unit used in known types of scanning particle mobility sizer (SMPS) apparatus. The DMA comprises a chamber having a sheath gas inlet 1 which is used to introduce a sheath gas flow into the DMA; a sheath gas outlet 2 for the sheath flow; an aerosol inlet 4; an aerosol outlet 5, and a pair of opposed electrodes 6 and 7 connected to a DC voltage supply. A flow maintaining system 3 comprising a pump and aerosol filters (not shown), and associated tubing 8 provide a steady flow of sheath gas through the chamber.
In operation, charged aerosol particles (preferably each having a single charge) are introduced into the DMA via the aerosol inlet 4 and move along the interior chamber of the DMA towards the end containing the aerosol outlet 5 and the sheath gas outlet 2. As a consequence of the voltage applied to the electrodes 6 and 7, the particles will be attracted towards the electrode 6, the extent of the attraction depending on the voltage and the electrical mobilities of the particles. At a given voltage and sheath gas flow, a proportion of the particles (particles having the same electrical mobility) will follow trajectory 9 and will pass out through aerosol outlet 5 from which they are directed to a CPC or electrometer where they are counted. By varying the voltage, particles having different electrical mobilities can be directed to the outlet 5. Because the electrical mobility of the particles is generally proportional to the size of the particles, it is possible to fractionate the aerosol particles according to size by varying the voltage applied to the electrodes 6 and 7. In general, the greater the size of the particles, the greater the voltage required to select particles and direct them to the outlet 5.
Differential mobility analyzers working on the above principles can be used very effectively to select particles of up to about 500 nm but, for larger particle sizes, problems do arise. In order to select particles of larger sizes, higher electrode voltages will be required and this places certain practical limits on the DMA. If the voltage is too high, corona discharges (or even complete electrical field breakdown) are likely to occur between the electrodes. This would be a particular problem for DMA devices with relatively narrow gaps between the electrodes (for example in miniaturized or portable devices).
An alternative to increasing the electrode voltage is to increase the length of the DMA chamber but this would lead to DMA chambers of impractical length and would further militate against miniaturization and the construction of portable hand-held DMA devices.
A further alternative to increasing the electrode voltage or increasing the length of the DMA chamber would be to reduce the flow rate of the sheath gas thereby enabling each particle to spend more time in the electric field. However, reducing the flow rate will also allow more time for the particles to undergo random movement by diffusion thereby leading to poorer resolution. This is illustrated by FIG. 2 below which is a schematic view of aerosol particle size distributions obtained at various sheath flow rates. In FIG. 2, the solid line corresponds to a size distribution obtained at a higher sheath flow rate Qsh1 and the dashed line represent a size distribution obtained for the same aerosol but with a lower sheath flow rate Qsh2<Qsh1. At the lower sheath flow rate, the observed size distribution is significantly greater and less well defined than the size distribution obtained at the higher sheath flow rate.
At present, therefore, there remains a need for a method of obtaining aerosol particle size distributions in a wide range of particle sizes using a stand-alone DMA device or a DMA as part of an SMPS device, and in particular a method which can be carried out using a miniaturised or portable SMPS device.